The present invention relates in general to video processing circuits and, more particularly, to an integrated video overlay circuit.
Video overlaying is a process in which a first video signal is at least partially covered by a second video signal. Video overlay circuits are generally used to add text or graphics to an existing video stream. For example, many television receivers overlay the incoming video signal with an overlaying signal to display channel identification or control information in the corners of the video picture. Such information is often locally generated in a cable television set top receiver. Video overlaying can be either full or partial, such that the overlaying video either obscures the incoming video or appears transparent, allowing the incoming video to partially show through the overlaying video.
In one prior art video overlay system, an overlaid video signal is generated by switching between a composite incoming video signal and a composite overlay video signal, each of which includes both chrominance and luminance components. However, the image displayed in the prior art system suffers from display errors caused by switching transients which generate chroma sidebands within the luma bandwidth that are incorrectly processed by a television receiver as luminance signals. The switching induces a change in the phase and amplitude of the output signal's chroma subcarrier, which results in ringing and other display artifacts when viewed on a television screen. If the switching times are slowed down to reduce the sidebands, the display artifacts are reduced but the bandwidth of the luminance portion is also reduced, which causes the edges of the overlaying image to appear soft, fuzzy and/or smeared.
Another prior art video overlay circuit decodes the incoming signal into either YUV or RGB component signals, switches in the corresponding overlay component signals, and then re-encodes the combined signals to produce a composite output signal. However, the decoding and encoding processes can alter or destroy information contained in the incoming signal. For example, filters in the decoder circuit effectively reduce the chroma bandwidth and result in chroma smearing when displayed. In addition, decoding often introduces phase errors in the burst lock circuit and amplitude errors in the automatic color control circuit of the decoder. Similar errors generated in the encoder circuit result in hue and saturation artifacts.
Hence, a need exists for an overlay circuit and method of overlaying a composite video signal which reduces display artifacts without increasing the cost of a television receiver.